Electrolytic slime of copper or lead contains large amounts of silver and gold, which have been conventionally recovered. In a known electrorefining process, silver electrolytic decomposition is performed using crude silver obtained from electrolytic slime or by dry refining of copper or lead as an anode to collect purified silver, whereas the anode slime formed in the silver electrolytic decomposition is dissolved with nitric acid to remove impurities and the residual gold, which is not dissolved, is cast as an anode to obtain purified gold by gold electrolytic decomposition.
In another known method, electrolytic slime of copper or lead is leached with hydrochloric acid containing an oxidizing agent, and gaseous SO.sub.2 is introduced into the leaching solution for the reductive precipitation of gold. Since the reductive process can reduce 80% of gold in the solution, the residual unreduced gold is recycled in the former step for re-reduction treatment.
A wet refining process for recovering gold and silver with high efficiency is known in Japanese Patent Laid-Open No. 46-4775 in which copper electrolytic slime is subjected to a complicated dissolution operation at a high temperature. For example, the copper slime is leached with sulfuric acid in an autoclave with application of pressure, the leaching residue is chlorinated for recovering gold in the solution by solvent leaching whereas the residue after chlorination is leached with ammonia to dissolve silver as an ammonia complex which is reduced with glucose or the like to form crude silver.
The above-mentioned electrorefining process has the following problems, that is, (1) the time required for gold refining is prolonged because of gold electrolysis after silver electrolysis, resulting in low gold productivity; and (2) the anode casting operation for electrolysis requires a lot of time and is automated with difficulty, resulting in low workability and productivity.
The method for reducing silver in the leaching solution with gaseous SO.sub.2 has the following problems, that is, (1) since the leaching solution contains large amounts of platinum and palladium, the reduction rate of gold must be suppressed to approximately 80% to maintain the quality and thus 20% of the residue must be subjected to retreatment, resulting in a low treating efficiency; and (2) it is difficult to control gaseous SO.sub.2, and the quality of gold will decrease significantly if the conditions change.
The conventional method for obtaining reduced silver by ammonia leaching has the following problems, that is, (1) since large amounts of impurities such as lead are contained in the slime, the removal treatment for such impurities is significantly expensive; (2) copper and lead contain large amounts of impurities regardless of a complicated series of steps from ammonium leaching to reduction.
For solving these problems in the conventional processes, it is an object of the present invention to provide a method for refining noble metals which is capable of refining with ease crude silver by a relatively simple treating process, and of reducing the refining time by simultaneous silver refining and gold recovery steps.